raaaarely. you generally have an established codebase of tens of thousands of lines of code that the business is based on... usually with little to no test cov

raaaarely. you generally have an established codebase of tens of thousands of lines of code that the business is based on... usually with little to no test coverage. Creating a parallel version of the code in the new language would take ages and be filled with errors because the lack of tests proving the behavior in the old one means you rarely know if you new version is "correct" or not ... and then you end up with annoyed customers but... all of that is moot because you can't get management to sign off on using so many resources for so long to do a thing that customers will never see and can't be marketed.